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WHO Warns Of Zika Virus's 'Alarming' And 'Explosive' Spread

In her first major address on the Zika outbreak, the head of the World Health Organization, Dr. Margaret Chan, said the mosquito-borne virus has gone from being "a mild threat to one of alarming proportions." Chan spoke Thursday in Geneva.

The apparent link between Zika and severe birth defects in children in Brazil still hasn't been definitively proven but Chan says the threat is so high that she's calling for an emergency committee to advise her on "the appropriate level of international concern and for recommended measures that should be undertaken in affected countries and elsewhere." There's no vaccine or treatment for Zika. Chan says eliminating mosquito breeding grounds is one of the most important defenses against the disease.

"We really need to concentrate on mosquito control," Chan said in a briefing to the Executive Committee of the WHO.

The Zika virus was first detected in the Americas in May 2015 in Brazil. It has since spread in Chan's words "explosively" to 22 countries and territories in the hemisphere. There have also been a handful of cases in the mainland United States, but all of those have been in people who recently returned from Zika-affected areas.

"Arrival of the virus in some places," Chan says, "has been associated with a steep increase in the birth of babies with abnormally small heads and in cases of Guillain-Barre syndrome [a neurological condition]."

Another top official at the briefing, Dr. Marcos Espinal, the director of Communicable Diseases and Health Analysis at the WHO's Pan American Health Organization, noted that those neurological conditions have so far only been found in Brazil. Espinal said this raises the possibility that other factors in addition to Zika may be causing the severe health problems. Some researchers have wondered whether someone who gets infected with Zika and dengue at the same time, for instance, may suffer worse effects.

Chan and WHO were sharply criticized for moving too slowly to address the 2014 Ebola outbreak in West Africa. Some public health experts say the world's health agency has not been moving fast enough on Zika. Georgetown professors Lawrence Gostin and Daniel Lucy, writing this week in JAMA, said, "WHO headquarters has thus far not been proactive, given potentially serious ramifications."

Chan's emergency committee is meeting on Monday to determine what measures should be taken both in the affected countries and elsewhere to address the outbreak.

Dr. Espinal said the virus is expected to spread to other parts of the world where dengue — which is spread by the same mosquito — is also present.

He added that he expects this outbreak to grow from the current estimate of 1 million cases to 3 to 4 million cases before it's brought under control.

Because symptoms are typically mild, the severity of the threat has not been established. But as Chan said in the briefing in Geneva: "Questions abound. We need to get some answers quickly."

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Jason Beaubien is NPR's Global Health and Development Correspondent on the Science Desk.